I liked the episode but agree that section was rough. I also don't understand the argument that a 6 UM somehow magically recoils less other than confirmation bias. Bore diameter does not have an effect on recoil. The only explanation I can think of in their conversation is that the rifles being shot side-by-side are not an apples to apples comparison. I can believe the felt recoil being better if one rifle has a better stock design, different muzzle device, or a higher rifle weight.
At the end of the day, it's the mass and acceleration of powder gasses/particles out of the muzzle and the mass and acceleration of the projectile out of the muzzle divided by the mass of the rifle to get the recoil acceleration. It's some super complex formula. My shoulder can definitely tell when I add 20 grains (creedmoor vs. UM increase) of Blackhorn 209 to my muzzleloader behind the same projectile in the same rifle.